Certain problems are inherent in the machining of small parts to be used in environments where a very small tolerance is available with respect to the surface characteristics of those parts. Referencing in particular the specific application for which this invention was developed, transducing heads for high density magnetic disk drives in data storage units are required to fly very close (about 10-15 microinches) to the magnetic surface in order for the disk drive to function properly. To close, and the head and slider will crash into the surface of the disk causing loss of data, damage to the substrate and potentially a damaged head, too far from the surface and the accuracy of the read/write capabilities of the head become diminished to the point of non-functionality. The disks which are coated with a magnetic media are rotating typically at a speed of 3600 rpm while the heads are reading or writing. A typical magnetic air-bearing slider containing transducing heads is described in U.S. Pat. No. 3,855,625 issued to Gariner et al. Methods for machining such air bearing sliders with a laser are described in U.S. No. 4,301,353, Swenaga, et al.
One problem overcome by this invention is peculiar to the construction of thin film head sliders. On the trailing edge of the slider are the transducing heads and relatively large metal leads to these heads and contacts which are produced on the flyers by a thin film technique. Thin film deposition of these elements allows for the construction of very high precision and very small heads. These elements are overlaid with an oxide layer to prevent corrosion and to maintain long functional lives as well as to prevent mechanical damage. As the laser is used to machine the end or trailing edge of the slider, it comes into proximity with these heads and lead structures. This can result in severe damage to or loss of one or more of the transducing heads at the trailing edge of the slider in the extreme case, or undetectable loss of the protective oxide coating in less extreme cases.
Another problem encountered in laser machining of sliders is the need for tight control of the pattern cavity depth. Conventional laser cutting and machining systems employ a laser which is fairly large and a set of movable optics for controlling the position of the beam that does the machining or cutting. Due to this arrangement, if an identical pattern of cuts are to be made on a series of pieces lined up in a row (or a continuous "bar"), as the optical assembly is moved from one end of the row to the other, the beam path length is increased or decreased and this changes the size of the beam as it is focused on the surface of the part. The change in beam size causes a change in the resulting cavity depth of the pattern being machined which affects in the case of the slider the flying height. Where the sliders are constructed from a single bar and the beam size changes significantly from one end of the bar to the other, the cavity depth of the sliders at one end of the bar will be significantly different from that of the sliders on the other end of the bar.
For the purposes of this application focus is defined as that distance from the objective or focusing lens of the optical system at which the laser beam is at its narrowest waist. For the purposes of this definition, the focusing or objective lens is considered to be a converging lens or lens system.